i18n: complete Simplified Chinese catalog, Chinese font fallback and a coverage xtask (#930)

Adds a complete Simplified Chinese (zh) catalog covering every English message, a font fallback for Chinese text, and `cargo xtask i18n --check`, which checks that the complete catalogs (ja, zh) carry every English message and that every command label is translatable.

Co-authored-by: Noly1990 <20500931+Noly1990@users.noreply.github.com>

xtask/src/main.rs

@@ -6,6 +6,7 @@
 mod assets;
 mod book;
 mod features;
+mod i18n;
 mod layers;
 mod licences;
 mod test;
@@ -20,6 +21,12 @@ commands:
   layers          enforce the crate dependency layering (plan/architecture.md §3)
   assets          check that every non-code asset file is attributed in ATTRIBUTION.md
   features        list SolidCraft's commands by area in docs/features.md (from the registry)
+  i18n [--check | --add-missing]
+                  report how complete the message catalogs in crates/ui-egui/src/locales are:
+                  which messages a locale has not translated and which command labels the English
+                  catalog cannot translate (so they stay English everywhere). --check fails when
+                  there is a gap, for CI; --add-missing writes the uncovered command labels into
+                  en.json under cmd.<command id> so only the translations are left to do
   oracle [--cli BIN]
                   replay the reference tests (tests/oracle/*/recipe.json), compare with each
                   case's expected.json and write docs/oracle.md (--cli: a solidcraft-cli
@@ -49,6 +56,7 @@ fn main() -> ExitCode {
         Some("layers") => cmd_layers(),
         Some("assets") => assets::run(&root()),
         Some("features") => features::run(&root()),
+        Some("i18n") => i18n::run(&root(), &rest),
         Some("oracle") => cmd_oracle(rest.iter().position(|a| *a == "--cli").and_then(|i| rest.get(i + 1)).copied()),
         Some("action-matrix") => {
             cmd_action_matrix(rest.contains(&"--bless"), rest.iter().position(|a| *a == "--results").and_then(|i| rest.get(i + 1)).copied())